The Synthesis, Structure and (FTIR-Spectro)Electrochemistry of W(CO)5 complexes of 4-oxo-4-(2,5-dimethyl-azaferrocen-1’-yl) butanoic and 5-oxo-5-(2,5-dimethylazaferrocen-1’-yl) pentanoic acids

Abstract

With the aim of developing new IR-detectable metal-carbonyl tracers for the amino function, we have synthesized W(CO)(5) complexes of 4-oxo-4-(2,5-dimethylazaferrocen-1'-yl)butanoic acid (2) and 5-oxo-5-(2,5-dimethylazaferrocen-1'-yl)pentanoic acid (3) by AlCl3-catalyzed Friedel-Crafts reaction Of W(CO)(5)-2,5-dimethylazaferrocene (1) with succinic or glutaric anhydride. Complexes 2 and 3 are thermally stable and display sharp, intense absorption bands of lungsten-coordinated CO ligands at ca. 1923 cm(-1). In the crystalline state, molecules of 2 and 3 are stabilized by a network of intra- and intermolecular hydrogen bonds, as shown by single-crystal X-ray structure analysis. Complex 2 was transformed into the corresponding N-succinimidyl ester 4. Its utility toward labeling of amino acids was tested in its reaction with glycine methyl ester. Corresponding glycine amide 5 was obtained in 82% yield and is an air/thermally stable bioconjugate exhibiting intense sharp absorption bands of the W-CO reporting group at ca. 1923 cm(-1). Cyclic voltammetry of 1, 2, 3, and acetyl derivative 6 shows the presence of two redox events in each case. The first redox couple is ascribed as an azaferrocene-centered oxidation-reduction, whereas the second, irreversible process at higher potential originates from a W(CO)(5)-centered oxidation. FTIR spectro-electrochemistry allowed us to monitor the spectroscopic changes accompanying the 1/1(.+), 2/2(.+), and 6/6(.+) redox transformations. Significant W-CO absorption band shifts were recorded in the course of these experiments. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009